Texturing of the bearing surface for a roller cone rock bit

ABSTRACT

Surface texturing is employed to modify the topography of one or more surfaces (radial or cylindrical) of the bearing system for a roller cone rock bit. The surface texturing results in a dimpled surface which retains additional lubricant helpful in reducing friction in the boundary and mixed lubrication regimes. Shot peening is disclosed as one method for texturing the desired surface.

PRIORITY CLAIM

This application is a divisional of U.S. patent application Ser. No.12/398,730, filed Mar. 5, 2009, which claims the benefit of U.S.Provisional Application for patent Ser. No. 61/036,785 filed Mar. 14,2008, the disclosures of both of which are hereby incorporated byreference.

TECHNICAL FIELD

The present invention relates generally to earth boring bits, and moreparticularly to roller cone rock bits.

BACKGROUND

A roller cone rock bit is the cutting tool used in oil, gas, and miningfields to break through the earth formation to shape a well bore. Loadand motion of the bit are transferred to the bearings inside three headand cone assemblies. For the bit where a journal bearing is employed,the main journal bearing is charged with as much as 80 percent of thetotal radial load. The main journal bearing is composed of the head (asthe shaft), the bushing, and the cone (as the housing). Due to high load(>10,000 lb) and low speed (70˜250 rpm), the bearing operates in theboundary or the mixed lubrication regime in which the head is notcompletely separated from the bushing. Load is supported by bothasperity contact and hydrodynamic pressure. The grease constrained amongthose asperities is the effective lubricating medium and its amount isquite limited. It is not unusual for the bearing to experience greasestarvation in the contact zone. Thereafter the bearing often undergoesscoring, scuffing, even catastrophic failure like galling or seizure. Itis desirable that more grease be trapped between the head and bushing atthe contact interface in order to reduce friction.

Reference is made to FIG. 1A which illustrates a partially broken awayview of a typical roller cone rock bit. FIG. 1A more specificallyillustrates one head and cone assembly. The general configuration andoperation of such a bit is well known to those skilled in the art.

The head 1 of the bit includes the bearing shaft 2. A cutting cone 3 isrotatably positioned on the bearing shaft 2 which functions as ajournal. A body portion of the bit includes an upper threaded portionforming a tool joint connection 4 which facilitates connection of thebit to a drill string (not shown). A lubrication system 6 is included toprovide lubrication to, and retain lubricant in, the journal bearingbetween the cone 3 and the bearing shaft 2. This system 6 has aconfiguration and operation well known to those skilled in the art.

A number of bearing systems are provided in connection with the journalbearing supporting rotation of the cone 3 about the bearing shaft 2.These bearing systems include a first cylindrical friction bearing 10(also referred to as the main journal bearing herein), ball bearings 12,second cylindrical friction bearing 14, first radial friction (thrust)bearing 16 and second radial friction (thrust) bearing 18.

With reference to FIG. 1B, there is shown an illustration of a partiallybroken away view of another typical roller cone rock bit. Reference 141refers to the axis of rotation for the cone 150 on the head shaft 130.Reference 106 refers to central rotating axis of the bit itself. Likereference numbers refer to like or similar parts. It will be noted herethat a tapered thrust bearing 16′ is used.

FIG. 2 illustrates a partially broken away view of FIG. 1A showing thebearing system in greater detail. The first cylindrical friction bearing10 is defined by an outer cylindrical surface 20 on the bearing shaft 2and an inner cylindrical surface 22 of a bushing 24 which has been pressfit into the cone 3. This bushing 24 is a ring-shaped structuretypically made of beryllium copper, although the use of other materialsis known in the art. The ball bearings 12 ride in an annular raceway 26defined at the interface between the bearing shaft 2 and cone 3. Thesecond cylindrical friction bearing 14 is defined by an outercylindrical surface 30 on the bearing shaft 2 and an inner cylindricalsurface 32 on the cone 3. The outer cylindrical surface 30 is inwardlyradially offset from the outer cylindrical surface 20. The first radialfriction bearing 16 is defined between the first and second cylindricalfriction bearings 10 and 12 by a first radial surface 40 on the bearingshaft 2 and a second radial surface 42 on the cone 3. The second radialfriction bearing 18 is adjacent the second cylindrical friction bearing12 at the axis of rotation for the cone and is defined by a third radialsurface 50 on the bearing shaft 2 and a fourth radial surface 52 on thecone 3.

An o-ring seal 60 is positioned between cutter cone 3 and the bearingshaft 2. A cylindrical surface seal boss 62 is provided on the bearingshaft. In the illustrated configuration, this surface of the seal boss62 is outwardly radially offset (by the thickness of the bushing 24)from the outer cylindrical surface 20 of the first friction bearing 10.It will be understood that the seal boss could exhibit no offset withrespect to the main journal bearing surface if desired. An annular gland64 is formed in the cone 3. The gland 64 and seal boss 62 align witheach other when the cutting cone 3 is rotatably positioned on thebearing shaft. The o-ring seal 60 is compressed between the surface(s)of the gland 64 and the seal boss 62, and functions to retain lubricantin the bearing area around the bearing systems and prevents anymaterials (drilling mud and debris) in the well bore from entering intothe bearing area.

With reference once again to FIG. 1B, the tapered thrust frictionbearing 16′ is defined between the first and second cylindrical frictionbearings 10 and 12 by a first conical surface 40′ on the bearing shaft130 and a second conical surface 42′ on the cone 150. The features ofFIG. 1B are otherwise generally the same as in FIGS. 1A and 2.

While the surfaces have in some instances been referred to ascylindrical or radial, and have been shown as linear, it will beunderstood that other surface geometries (for example, non-lineargeometries neither parallel with nor perpendicular to the axis 141 ofcone rotation, such as toroidal or otherwise curved) as are known tothose skilled in the art may used for the bearing and thrust surfaces.

Load in the bearing system is supported by both asperity contact andhydrodynamic pressure. Lubricant is provided in the first cylindricalfriction bearing 10, second cylindrical friction bearing 14, firstradial friction bearing 16 (or tapered thrust bearing 16′) and secondradial friction bearing 18 between the implicated cylindrical and radialsurfaces using the system 6. However, it is not unusual for the bearingto experience grease starvation in these surface contact zones of thebearing system. This can result in scoring, scuffing, and evencatastrophic failure like galling or seizure. There is a need to retainlubricant in position trapped between the implicated and opposedcylindrical and radial surfaces of the bearing system.

Reference is made to the following prior art documents: U.S. Pat. Nos.3,839,774 (Oct. 8, 1974), 4,248,485 (Feb. 3, 1981) and 5,485,890 (Jan.23, 1996): U.S. Publication 2005/0252691 (Nov. 17, 2005); and PCTPublication WO 2007/146276 (Dec. 21, 2007), the disclosures of which arehereby incorporated by reference.

SUMMARY

To address issues of grease starvation and possible bearing failure, itis desired to increase the amount of lubricant that can be maintained inthe surface contact zones of the bearing system. In an effort tointroduce more lubricant into these surface contact zones, the surfacetopography of the bearing system is modified in the manner describedbelow.

Surface texturing is employed to modify the topography of one or moresurfaces (without limitation, for example, radial or cylindrical) of thebearing system for a rock bit. Innovative methods and apparatus aredescribed with respect to certain features of surface texturing and itsbeneficial effect on reducing bearing friction and prolonging bit life.These features address deficiencies of the prior art with respect to theconfiguration and operation of the main journal bearing surfaces as wellas the pilot pin bearing surfaces and thrust bearing surfaces.

Due to heavy load and low velocity, the head shaft and the bushing ofthe main journal bearing are in contact at the loading side of thebearing system. This metal-to-metal contact dominates the frictionalbehavior of the bearing system. The friction coefficient can normallyreach over 0.1, which generates enormous heat and can lead to bearingand seal failure. To improve the bearing life, the friction has to bereduced. In a mixed lubrication regime, there are two means to createbetter lubrication in these surface contact areas: supply more grease orgenerate greater hydrodynamic pressure.

In accordance with an embodiment, the topography of the head bearingsystem is modified by surface texturing technology. A surface texture isintroduced, either on the head side or on the bushing (or cone) side (orboth), of the bearing system for the roller cone rock bit. The surfacetexturing disclosed herein includes dimples which retain additionallubricant and are thus helpful to reduce the friction in the boundaryand mixed lubrication regimes.

Surface texturing as described herein creates specially patterneddimples on one or more surfaces of the bearing system. Reference is onceagain made to FIGS. 1A and 2 for an identification of possible texturedsurfaces in the bearing system in accordance with embodiments describedherein. In one implementation, the surface texturing is provided on anouter cylindrical surface 20 of the bearing shaft 2 which forms part ofthe first cylindrical friction bearing 10. In another implementation,the surface texturing is provided on an inner cylindrical surface 22 ofthe bushing 24 which has been press fit into the cone 3 and which formspart of the first cylindrical friction bearing 10. In yet anotherimplementation, the surface texturing is provided on an outercylindrical surface 30 of the bearing shaft 2 which forms part of thesecond cylindrical friction bearing 14. In still another implementation,the surface texturing is provided on an inner cylindrical surface 32 ofthe cone 3 which forms part of the second cylindrical friction bearing14. In another implementation, the surface texturing is provided on afirst radial surface 40 (or conical surface 40′) of on the bearing shaft2 which forms part of the first radial friction bearing 16 (bearing16′). In yet another implementation, the surface texturing is providedon a second radial surface 42 (or conical surface 42′) of the cone 3which forms part of the first radial friction bearing 16 (bearing 16′).In still another implementation, the surface texturing is provided on athird radial surface 50 of the bearing shaft 2 which forms part of thesecond radial friction bearing 18. In yet another implementation, thesurface texturing is provided on a fourth radial surface 52 of the cone3 which forms part of the second radial friction bearing 18. Anycombination of the foregoing textured surfaces, with untexturedsurfaces, may also be used.

The dimples of the surface texturing behave as lubricant reservoirswhich permeate the lubrication into the inter-space of metal asperities.Meanwhile, higher hydrodynamic pressure is generated on the dimple area.Both functions facilitate an improvement in bearing system lubricationwith a reduction in friction. It is preferred that the dimples ofsurface texture cover between 60-100% of the contact bearing surface ofinterest. Even more preferably, the dimples should cover between 70-90%of the surface of interest. In an implementation, the dimples coversubstantially 100% of the surface of interest.

Embodiments herein utilize any one or more of a variety of methods tocreate surface texturing including: machining, chemical etching, lasertexturing, deep rolling, vibratory finishing, etc. Controllability,uniformity, cost, coverage area, dimple size, dimple depth, and dimpleshape are the factors which determine which method is selected to formthe texturing.

In a preferred implementation, shot peening is used to create thedimples of the surface texturing. More specifically, a two-step shotpeening process is used. In accordance with this two-step process, in afirst step the bearing system surface to be treated is exposed to afirst shot peening action wherein the surface is bombarded at a firstintensity level by small spherical media (the “shot”) of a first averagesize. In a second step the same bearing system surface being treated isexposed to a second shot peening action at a second intensity levelwherein the surface is bombarded by small spherical media (the “shot”)of a second average size. Preferably, the second intensity level isreduced from the first intensity level. Preferably, the second averagesize is smaller than the first average size.

In a preferred implementation, each step of the two-step shot peeningprocess is effectuated to achieve a peened coverage area of between60-100%. When peened coverage areas of less than 100% are used in eachstep, the goal is to achieve a final peened coverage area with respectto the treated surface of at least 60%, and more specifically 70-90% andeven more preferably which approaches or reaches substantially 100%.

It will further be understood that the shot peening process couldutilize more than two separate peening actions. For example, athree-step, four-step, or more-step process could be used. Each stepwould preferably utilize different average sized media and differentintensity levels.

Disclosure is also made of an intermittent type of texturing for bearingsurfaces wherein not all of a given surface receives the surfacetexturing treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a partially broken away view of a typical rollercone rock bit;

FIG. 1B illustrates of a partially broken away view of another typicalroller cone rock bit;

FIG. 2 illustrates a partially broken away view of FIG. 1A showing thebearing system in greater detail;

FIG. 3 illustrates an exemplary shot peening impact pattern;

FIG. 4 illustrates, in cross-section, a surface of interest which hasbeen treated by the shot peening of FIG. 3 at or about 100% coveragearea;

FIG. 5 illustrates, in cross-section, a surface of interest which hasbeen treated by additional shot peening with a coverage area of lessthan 100%;

FIG. 6 illustrates an exemplary impact pattern with respect to executionof the additional shot peening;

FIGS. 7, 8 and 9 illustrate, in cross-section, a surface of interestwhich has been treated by an additional shot peening process;

FIGS. 10 and 11 are images illustrating topography comparisons betweensurfaces that have subjected to a two-step shot peening process;

FIG. 12 is an image illustrating the topography of a regular machinedsurface;

FIG. 13 is an image illustrating the topography of a two-step shotpeened surface;

FIGS. 14, 15 and 16 illustrate the beneficial effect on the frictioncoefficient which accrues from surface texting;

FIG. 17 illustrates a partially broken away view of a roller cone rockbit when operating in a formation;

FIG. 18 illustrates a cross-section taken through a journal bearing; and

FIG. 19 illustrates an alternative form of intermittent surfacetexturing.

DETAILED DESCRIPTION OF THE DRAWINGS

Surface texturing is employed to modify the topography of one or moresurfaces (radial or cylindrical) of the bearing system for a roller conerock bit. The surface texturing results in a dimpled surface whichretains additional lubricant helpful in reducing friction in theboundary and mixed lubrication regimes. Surface coverage area for thedimpled texture should exceed at a minimum 60%, more preferably bebetween 70-90%, and even more preferably approach or reach approximately100%.

Reference is once again made to FIGS. 1 and 2 for an identification oftextured surfaces in the bearing system to which this surface texturingis applied.

Turning first to the first cylindrical friction bearing 10, surfacetexturing is provided on one or the other or both of the outercylindrical surface 20 on the bearing shaft 2 and the inner cylindricalsurface 22 of the bushing 24 which has been press fit into the cone 3,these surfaces forming the first cylindrical friction bearing 10 (ormain journal bearing).

With respect to the second cylindrical friction bearing 14, surfacetexturing is provided on one or the other or both of the outercylindrical surface 30 of the bearing shaft 2 and the inner cylindricalsurface 32 of the cone 3.

For the first radial friction bearing 16, surface texturing is providedon one or the other or both of the first radial surface 40 (or surface40′) of the bearing shaft 2 and the second radial surface 42 (of surface42′) of the cone 3.

Lastly, for the second radial friction bearing 18, surface texturing isprovided on one or the other or both of the third radial surface 50 ofthe bearing shaft 2 and the fourth radial surface 52 of the cone 3.

Any combination of the foregoing textured surfaces, with desireduntextured surfaces, may also be used.

The dimples of the surface texturing behave as lubricant reservoirswhich permeate the lubrication into inter-space of metal asperities.Meanwhile, higher hydrodynamic pressure is generated on the dimple area.Both functions facilitate an improvement in bearing system lubrication.

Any one or more of a variety of methods can be used to create thedimpled surface texturing including: machining, chemical etching, lasertexturing, deep rolling, vibratory finishing, shot peening, etc.Controllability, uniformity, cost, coverage area, dimple size, dimpledepth, and dimple shape factors which influence which method is selectedfor the surface texturing process.

The dimpled surface texture should be random and with uniform coverage.Preferably, different sized dimples should be present and should berandomly distributed across the surface. A finished coverage area ofsubstantially 100% on the surface of interest is preferred. If theoriginal surface is obliterated entirely by overlapped dimple texturing,then it can be said that 100% coverage area has been achieved.Additionally, the finished surface texture should lack any sharp edgeswhich could contribute to undesirable metal-to-metal contact in thebearing system. It will be recognized, however, that benefits accruefrom finished coverage areas on the surface of interest in excess of60%, or more preferably between 70-90%, and on up approaching 100%.

In a preferred implementation, shot peening is chosen to form thedimpled surface texture through topology modification. Shot peeningadvantageously has characteristics of randomicity but with uniformity ofcoverage. Shot peening is also a controllable process so that only thosedesired surfaces will have a modified surface topography (this allowsfor a machined surface to exist adjacent to a peened surface). As knownto those skilled in the art, shot peening is a cold working process inwhich the surface of a part is bombarded by small spherical media calledshot. Each shot leaves a tiny dimple on the surface caused by impact.Shot peening is more widely used to create compressive stress so as toreduce fatigue crack. Inspired by the view that tiny dimples aregenerated on the surface, shot peening is employed as described hereinfor a different purpose in creating a dimpled surface texture which canconstrain more lubricant in the bearing surface contact zone(s) andgenerate increased hydrodynamic pressure which better separates thebearing surfaces.

Any one or more of the surfaces 20, 22, 30, 32, 40, 42, 50 and 52described above can be subjected to the shot peening treatment. In apreferred implementation, a two-step (dual) shot peening process isutilized on the surface(s) of interest.

In a first step, shot peening is performed on the surface using a firstshot media at a first shot intensity. The shot peening action of thefirst step is performed for a first period of time in order to obtain adesired coverage area. The shot media is preferably cast steel which inan exemplary implementation has a first average size of 0.011 inches,and the intensity of the first step is 0.007˜0.010 C (measured by Almenstrip). Alternatively, the shot media is glass bead for softer surfacessuch as the inner cylindrical surface 22 of the first cylindricalfriction bearing 10 on the bushing 24 (with an intensity of the firststep being 0.008˜0.012 N). In an exemplary implementation, the glassmedia has an average size of 0.006 inches. The distinction between ahard surface and a soft surface may be made based, for example, onwhether the hardness of the material exceeds a certain threshold (suchas, for example, a hardness of HRC 45). In the implementation describedabove for the main journal bearing, the journal of hardened steelmaterial has a hardness of HRC 58-62, while the bushing of berylliumcopper has a hardness of about HRC 38.

Preferably, the peened coverage area resulting from completion of thisfirst shot peening step after the first period of time is between 60%and 100%. Coverage in excess of 100% may also be provided. Coverage isdefined as the extent (in percent) of complete texturing (for example,dimpling) of the surface by the process step. Thus, with 100% coveragethe original surface texture of the surface which has been peened hasbeen obliterated entirely by the first shot peening step. Coverage inexcess of 100% is obtained by extending the exposure time to peeningbeyond that time which is required to achieve 100% coverage. Forexample: a 200% coverage would be achieved by shot peening the surfacefor twice the amount of time necessary to obtain a 100% coverage.

FIG. 3 illustrates an exemplary impact pattern with respect to executionof the first shot peening step with at or about 100% coverage area.

FIG. 4 illustrates, in cross-section, a surface of interest which hasbeen treated by the first shot peening step.

Conversely, FIG. 5 illustrates, in cross-section, a surface of interestwhich has been treated by the first shot peening step with a coveragearea of less than 100% (i.e., for a shorter period of time).

In a second step, shot peening is performed on the surface (FIG. 4 or 5)resulting from completion of the first step using a second shot media ata second intensity. The shot peening action of the second step isperformed for a second period of time in order to obtain a desiredcoverage area. The shot media is preferably cast steel having a secondaverage size of 0.011 inches (which is smaller than the first averagesize), and the intensity of the second step is 0.007˜0.010 A (measuredby Almen strip). Alternatively, the shot media is smaller glass bead forsofter surfaces such as the inner cylindrical surface 22 of the firstcylindrical friction bearing 10 on the bushing 24. Preferably, thepeened coverage area resulting from completion of this second shotpeening step is between 60% and 100%.

FIG. 6 illustrates an exemplary impact pattern with respect to executionof the second shot peening step with less than 100% coverage area.

FIG. 7 illustrates, in cross-section, a surface of interest which hasbeen treated by the second shot peening step (when starting from thefirst step result shown in FIG. 4) for a second period of time necessaryto obtain substantially 100% coverage area.

FIG. 8 illustrates, in cross-section, a surface of interest which hasbeen treated by the second shot peening step (when starting from thefirst step result shown in FIG. 5). In this case, the second step hasless than 100% coverage (due to exposure for a shortened second periodof time).

FIG. 9 illustrates, in cross-section, a surface of interest which hasbeen treated by the second shot peening step (when starting from thefirst step result shown in FIG. 5). In this case, the second step has100% coverage through selection of the requisite second period of time.

No matter what coverage area percentage is accomplished with the secondshot peening, it is preferred that the second shot peening step at aminimum compact, as shown in FIG. 7, any sharp edges present in thesurface texture resulting from completion of the first shot peening step(see FIGS. 4 and 5). This will result in an improved surface texturingfinish wherein the possibility of metal-to-metal contact in the bearingsystem is reduced.

It is preferred that following completion of the shot peening treatment(both or more steps) of the surface of interest, that substantially 100%coverage area for combined first and second step surface treatment withdimpling be achieved. However, there are advantages to coverage areas ofgreater than 60%, 70-90%, and greater than 90%.

Although a two-step process is described, it will be understood that theshot peening process could utilize more than two separate peeningactions. For example, a three-step, four-step, or more-step processcould be used. Each step would preferably utilize different averagesized media and different intensity levels.

For softer materials, for example at or below a hardness HRC45, only oneshot peening step action may be necessary. Harder materials, however,benefit from the performance of two or more shot peening actions asdescribed above.

It will be understood that the cross-sectional surface textureillustrations shown herein are schematic and exemplary in nature. Theillustrated regularity and periodicity of the dimple shape and locationshown in the FIGURES is not necessarily an accurate illustration of whatan actual shot peened surface would look like in cross-section butrather is representative of the results achieved with the two stepprocess. One skilled in the art will understand the topologies whichresult from each of the first and second steps given differentrespective first and second periods of time for the peening action.

It is known in the prior art to provide radial and cylindrical bearingsystem surfaces having a roughness of 8 to 16 microinches Ra. This wouldcomprise a typical polished bearing surface of standard use (see, also,FIG. 12). As a result of the completion of the surface treatment processdescribed herein, however, the shot-peened bearing surface of interestwill have a surface finish roughness greater than 20 microinches Ra(see, also, FIG. 13).

Reference is now made to FIG. 10 which is an image illustratingtopography comparisons between surfaces that have been processed inaccordance with the two-step shot peening process described above tohave a surface roughness of greater than 20 microinches Ra (see, thebearing shaft on the left) and surfaces with conventional surfaceroughness of 8 to 16 microinches Ra (see, bearing shaft on the right andbushing inner surface at center). FIG. 11 is an image illustratingtopography comparisons between surfaces that have been processed inaccordance with the two-step shot peening process described above tohave a surface roughness of greater than 20 microinches Ra (see, thebearing shaft on the left and bushing inner surface at center) andsurfaces with conventional surface roughness of 8 to 16 microinches Ra(see, bearing shaft on the right).

FIG. 12 is an image illustrating the topography of a regular machinedsurface such as would be used in the prior art for a bearing systemsurface.

FIG. 13 is an image illustrating the topography of a two-step shotpeened surface as produced in accordance with the description above.

A conventional machined shaft with a conventional machined bushing (seesurface of FIG. 12), a shot-peened shaft (see surface of FIG. 13) with aconventional machined bushing, and a conventional machined shaft with ashot-peened bushing, were tested on a bearing test rig under the sameoperating conditions. FIGS. 14 and 15 illustrate results of that testingand show the beneficial effect surface texting (in general) and two-stepshot peening (in particular) in accordance with the process describedabove has on the friction coefficient in these three bearing systemconfigurations. The existence of the small dimples of the producedsurface texture generates hydrodynamic pressure, stabilizes or reducesfriction. A shaft and bushing system with a regular machined finish (asknown in the art) is exposed to more asperity-to-asperity contact sothat the friction in this bearing system configuration shows a largevariation. For a shot-peened shaft and/or bushing system (twoimplementations illustrated), however, the micro-dimples provide morereservoirs for grease to lubricate the rubbing surfaces. Meanwhile, thegrease contained in the dimples will generate hydrodynamic pressure toseparate the friction couple better. Therefore, the friction tends to bemore stable or reduced.

FIGS. 14 and 15 differ only in the manner with which the illustrateddata is identified and presented.

Reference is now made to FIG. 16 which also illustrates the effect shotpeening in accordance with the two-step process described above has onthe friction coefficient in these three bearing configurations. FIG. 16illustrates the same information as presented in FIGS. 14 and 15, butthe presentation is made in a different way. The FIG. 16 illustrationloses some information shown in FIGS. 14-15 concerning frictionvariation in the regular head and bushing system and its deduction in ashot-peened head and bushing system. Nonetheless, the frictionalbenefits of the surface textured finishes for the bearing system asdescribed herein are evident.

In summary, a surface textured head bearing is presented for use in arock bit. Tiny dimples are created by a two-step shot peening process onone or more surfaces of interest in connection with the bearing system(for example, in the main journal bearing). The dimples of randomdistribution and non-uniform size are formed over the surface ofinterest (at least 60% coverage area) and work as reservoirs toconstrain more lubricant in the surface contact zone. Hydrodynamicpressure is generated in the dimple area and the bearing friction isreduced. Correspondingly, the bearing working condition is improved.

Reference is now made to FIG. 17 which illustrates a partially brokenaway view of a roller cone rock bit when operating in a formation.During operation of the bit, forces F are exerted through the cuttingelements of cone onto the journal. However, it will be noted that theforces are exerted onto a loading zone of the bearing surface which issmaller than the overall circumferential surface. For example, theloading zone may comprise only one-quarter (about) 90° of thecircumferential surface of the bearing.

Reference is now made to FIG. 18 which illustrates a cross-section takenthrough the journal bearing (for example, through the first cylindricalfriction bearing 10 or main journal bearing) perpendicular to the axis141 of cone rotation. The reference arrows A-A in FIG. 17 illustrategenerally the angle of the view, but do not specifically identify wherethe cross-section was taken. It will be seen that the loading zone isapproximately one-quarter (about 90°) of the circumferential surface ofthe bearing surface (journal surface for the main bearing). The forces Ffrom engaging the formation (FIG. 17) are exerted on the bearing surfaceprimarily (if not exclusively) in this area. It is accordingly thisloading zone of the bearing surface which needs to be protected againstfriction damage.

In one embodiment, it is only this loading zone of the bearing surfacewhich receives the surface texturing previously described. Thus, thesurface of the bearing in the region of the arc identified as loadingzone would receive texturing while the remaining surfaces would receiveno texturing. It will accordingly be noted that this illustrates adifferent form of coverage area percentage than that which waspreviously explained. Coverage area in connection with thisimplementation will be referred to as intermittent coverage of thebearing surface and can also be expressed in terms of a percentage. Inthis case, the percentage (for example, about 25% for the one-quartersurface coverage example provided) refers to the portion of the surfacewhich receives any form of texturing, versus the portion of the surfacewhich receives no texturing at all.

Reference is now made to FIG. 19 which illustrates the bearing surface,unwrapped, from point 0 to point 1 of the arc B-B shown in FIG. 18.Within the load zone (right hand side of the unwrapped surface), thereis provided surface texturing of an intermittent nature. As discussedabove, all of the surface in the load zone can be surface textured inthe manner described herein. FIG. 19 specifically illustrates analternative form of intermittent surface texturing. One or more distinctand separated surface textured regions can be provided within the loadzone on the bearing surface. Each of these regions can be formed usingthe surface texturing techniques described herein. The regions may takethe form of strips, as shown in FIG. 19, which extend parallel to theaxis of cone rotation. Even further, these strips can be aligned withthe locations where forces F are being exerted against the journalbearing. The circumferential area to the left of the unwrapped surfacereceives no surface texturing treatment at all. Furthermore, areas ofthe surface between the regions receive no surface texturing treatmentat all. Again, as discussed above, coverage area in connection with thisimplementation (intermittent coverage of the bearing surface) isexpressed in terms of a percentage. In this case, the percentage (forexample, about 12-15% for the textured regions within the one-quartersurface loading zone example provided) refers to the portion of thesurface which receives any form of texturing, versus the portion of thesurface which receives no texturing at all.

Embodiments of the invention have been described and illustrated above.The invention is not limited to the disclosed embodiments.

What is claimed is:
 1. A roller cone drill bit, comprising: a journalbearing structure having a bearing system including a bearing surface;at least a portion of the bearing surface having a surface texture,wherein the surface texture is formed by shot peening in a two-stepprocess including a first shot peening action wherein the portion of thebearing surface is bombarded at a first intensity level by first smallspherical media of a first average size and a subsequent second shotpeening action wherein the same portion of the bearing surface isbombarded at a second intensity level by second small spherical media ofa second average size, the first and subsequent second shot peeningactions creating lubricant reservoirs on the same portion of the bearingsurface, the lubricant reservoirs being adapted to contain lubricant forthe bearing system; and wherein the first intensity level is greaterthan the second intensity level and the first average size is largerthan the second average size.
 2. The roller cone drill bit of claim 1wherein the bearing surface is a thrust surface.
 3. The roller conedrill bit of claim 2 wherein the thrust surface is of a radial kind 4.The roller cone drill bit of claim 1 wherein the bearing surface is ajournal surface.
 5. The roller cone drill bit of claim 4 wherein thejournal surface is of a cylindrical kind
 6. The roller cone drill bit ofclaim 1 wherein the journal bearing structure includes a bushing and theportion of the bearing surface is one of an inside or outside surface ofthe bushing.
 7. The roller cone drill bit of claim 1 wherein the journalbearing structure includes a shaft and the bearing surface is an outersurface of the shaft.
 8. The roller cone drill bit of claim 1 whereinthe portion of the bearing surface is less than an overallcircumferential surface of the bearing surface.
 9. The roller cone drillbit of claim 8 wherein the portion of the bearing surface is a loadbearing portion of the bearing surface.
 10. The roller cone drill bit ofclaim 8 wherein the surface textured portion of the bearing surface isformed as a plurality of strips on the bearing surface.
 11. The rollercone drill bit of claim 10 wherein the surface textured portion of thebearing surface is a load bearing portion of the bearing surface and thestrips align with load forces applied against the bearing surface. 12.The roller cone drill bit of claim 1 wherein the first intensity levelis in a range of 0.007-0.010 inches measured on a C-type Almen strip andthe second intensity level is in a second range of 0.007-0.010 inchesmeasured on an A-type Almen strip.
 13. A drill bit, comprising: a shaftincluding a sealing surface and a bearing surface; a roller cone havingan annular gland and being rotatably mounted to the shaft such that abushing on the roller cone aligns with the bearing surface and theannular gland aligns with the sealing surface, the bearing surface ofthe shaft or an inner surface of the bushing having a surface textureincluding a plurality of dimples; wherein the plurality of dimples areformed by shot peening in a first shot peening action the bearingsurface of the shaft or the inner surface of the bushing at a firstintensity level by a first small spherical media of a first average sizeand shot peening in a second subsequent shot peening action the samebearing surface of the shaft or inner surface of the bushing at a secondintensity level by a second small spherical media of a second averagesize; wherein the first intensity level is greater than the secondintensity level and the first average size is larger than the secondaverage size; and wherein the first and second subsequent shot peeningactions create lubricant reservoirs on the bearing surface of the shaftor the inner surface of the bushing, the lubricant reservoirs beingadapted to contain lubricant.
 14. The drill bit of claim 13 wherein,following completion of the second shot peening action, more than 60% ofthe bearing surface of the shaft or of the inner surface of the bushinghas been textured with dimples.
 15. The drill bit of claim 14 wherein,following completion of the second shot peening action, 70-90% of thebearing surface of the shaft or of the inner surface of the bushing hasbeen textured with dimples.
 16. The drill bit of claim 14 wherein,following completion of the second shot peening action, substantially100% of the bearing surface of the shaft or of the inner surface of thebushing has been textured with dimples.
 17. A drill bit, comprising: ashaft including a sealing surface and a bearing surface; a roller conehaving an annular gland and being rotatably mounted to the shaft suchthat a bushing on the roller cone aligns with the bearing surface andthe annular gland aligns with the sealing surface; a plurality ofdimples formed on the bearing surface of the shaft or an inner surfaceof the bushing by: shot peening in a first shot peening action thebearing surface of the shaft or the inner surface of the bushing at afirst intensity level by a first small spherical media of a firstaverage size; shot peening in a second subsequent shot peening actionthe same bearing surface of the shaft or inner surface of the bushing ata second intensity level by a second small spherical media of a secondaverage size; wherein the first intensity level is greater than thesecond intensity level and the first average size is larger than thesecond average size; wherein the first and second subsequent shotpeening actions form lubricant reservoirs on the bearing surface of theshaft or the inner surface of the bushing, the lubricant reservoirsbeing adapted to contain lubricant for a drill bit; and wherein theplurality of dimples provide intermittent coverage.
 18. The drill bit ofclaim 17 wherein the intermittent coverage is provided on a region whichis less than an overall circumferential surface for the bearing surfaceor for the inner surface of the bushing.
 19. The drill bit of claim 17wherein the intermittent coverage is provided on a region which is aload bearing portion of the bearing surface or of the inner surface ofthe bushing.
 20. The drill bit of claim 19 wherein the intermittentcoverage is formed as strips on the bearing surface or on the innersurface of the bushing.
 21. The drill bit of claim 20 wherein the regionis a load bearing portion of the bearing surface or of the inner surfaceof the bushing and the strips align with the load forces applied againstthe bearing surface or against the inner surface of the bushing.